1. Field of invention
The present invention relates to a soft X-ray source and a method for manufacturing the same, and more particularly, to a soft X-ray source to be used in an X-ray lithographic apparatus which is suitable for producing semiconductor devices and which has a high-power and highly stable X-ray output.
2. Description of the prior art
Prior to the emergence of X-ray lithography so-called photolithography was the preferred lithographic technique. Photolithography employs an ultraviolet ray emitted from a high pressure mercury vapor lamp and the like. Since a minute pattern on the order of a submicron is desired, photolithography can no longer maintain their proud standpoints because of the diffraction effect and the diffusion effect of an ultraviolet ray in the photo resist. X-ray lithography, on the other hand, employs rays which have a shorter wave length than ultraviolet rays.
In X-ray lithography a mask adapted to employ a shadow printing technique similar to that used in photolithography is placed between an X-ray source and an object to be exposed, and then X-ray flux is irradiated over the entire area of the mask. An X-ray-sensitive material film, namely an X-ray resist film, formed on the object is thereby selectively exposed to the X-ray, and a submicron pattern formed on the mask can be transferred to the object. An X-ray provides a greater penetrating power to a material than the electron beam or the photon and hence is not susceptible to scattering or reflection depending on the kinds of materials. Therefore, the X-ray lithography allows an increase in thickness of a resist, while retaining the desired resolution, and this leads to an improvement in reliability of an etching mask in a subsequent etching process. For a broad description of X-ray lithography, see U.S. Pat. No. 3,743,842 and "PROCEEDING OF THE IEEE" VOL. 62, NO. 10, OCTOBER 1974 from pages 1361 to 1387.
When semiconductor devices are manufactured by employing such an X-ray lithographic apparatus, if it is intended to enhance the production efficiency by reducing the exposure time to increase the yield of semiconductor devices per unit time, then it is necessary to get the soft X-ray at a high output.
For obtaining soft X-rays having high output and high density, water-cooled rotating X-ray sources are used as described, for example, by Hughes in SOLID STATE TECHNOLOGY (May 1977), at pages 39-42. The X-ray sources in the prior art are made of aluminum or comprised of a substrate made of copper or copper alloy through which water is circulated and a surface film made of aluminum formed on the substrate and emitting Al-K X-ray line having 8.3A wavelength. However, the melting point of the aluminum is as low as 660.degree. C., so that if the surface of the soft X-ray source is bombarded with a high energy electron beam for the purpose of obtaining higher output X-rays, then the aluminum or aluminum film will be molten or recrystallized, resulting in damage to the aluminum surface, and as a result, the X-ray output cannot be enhanced. For example, the recrystallizing temperature of aluminum which has a purity of 99.9% is 200.degree. C., or lower, and as a result of measurement it has been confirmed that in the case of the water-cooled rotating X-ray source, the X-ray output begins to decrease at a range of 10 kW, or lower of the electron beam energy.
In order to remove this difficulty silicon, whose melting point is as high as 1410.degree. C., has been used as the soft X-ray source. The Si-K X-ray line which has a 7.1A wavelength has been used in X-ray lithography.
According to this method, it is considered that an effective output of X-rays will be enhanced by 2.about.3 times that which is obtained when aluminum is used for a soft X-ray source, because silicon has a performance which is more than twice as high as aluminum with respect to a melting point ratio, and because the transmittivity of X-rays through an X-ray output window made of beryllium of normally about 20 .mu.m in film thickness and an X-ray exposure mask made of silicon of normally about 3 .mu.m in film thickness, is improved by about 30.about.50% in comparison to aluminum.
However, in the past, mainly due to problems in working, silicon has not been used as a soft X-ray source, particularly in a water-cooled rotating X-ray source.
For instance, in case where aluminum is employed, it is possible to work it into a cylindrical form through the conventional machining process, and aluminum can be clad on a copper alloy having a high mechanical strength and then press-worked to form a double layer structure.
Since silicon is a brittle material, such a working process cannot be employed, but the method would be employed, in which, for instance, a copper alloy having a good thermal conductivity is worked into a cylindrical form and on its surface is formed a silicon film as by an evaporation process, a sputtering process or an ion-planting process.
However, according to the evaporation process or the sputtering process, the adhesion force between silicon and a copper or copper alloy is not sufficiently strong, and so, peeling off is apt to occur at these portions due to thermal deformation caused by electron beam bombardment. Furthermore, while the adhesive force itself of a silicon film formed by the ion-planting process is naturally sufficiently strong, the adhesion velocity in the case of making silicon film onto a substrate of a cylinder is so low that only an adhesion velocity of about 200A/hr at the most is attainable, and therefore, if it is desired to make a silicon film of several microns or more adhere under such conditions it would take several tens hours and thus a silicon film is practice not available. In addition, if the thickness of the silicon film in this case is too thin, an electron beam having high energy would penetrate through the silicon film, and may generate hard X-rays from the underlying metal.
For instance, if a copper alloy containing chromium is used as the underlying metal or the substrate, then hard or nearly hard X-ray of 1.38 A wavelength are generated from copper at an electron energy of about 9 kV or higher, and of 2.07 A wavelength from chromium at an electron energy of about 6 kV or higher.
Therefore, it is necessary to suppress the generation of hard X-rays having high energy as much as possible because they will damage the semiconductor devices when they are exposed to them. For this reason also the silicon film cannot be made too thin.
In addition, since the generation efficiency of the Si-K X-ray line is proportional to the 1.67-th power of the accelerating voltage for the electron beam, when the silicon film is too thin and hard X-rays are liable to be generated, one must use the apparatus with a reduced accelerating voltage for the electron beam, and consequently, the X-ray output would be greatly lowered.
Furthermore, since the thermal conductivity of silicon is equal to 0.2 (cal/cam.deg.sec) which is smaller than the thermal conductivity of aluminum of 0.5 (cal/cm.deg.sec), the use of silicon is less advantageous than aluminum with respect to thermal dissipation. Thus if, thermal conductivity is represented by .lambda. and film thickness by d, thermal dissipation is considered to be proportional to .lambda./d, so that in order to obtain a value of .lambda./d as high as that of aluminum, the thickness of the silicon film must be 1/3 or less times the thickness of the aluminum film. Thus, if one tries to prevent a temperature rise at the surface of the silicon film because of the enhancement of the colliding electron beam energy for the purpose of obtaining a high output, by improving the thermal dissipation, then it is necessary to make the silicon target film as thin as possible.
On the other hand, however, as described above with respect to the prior art target structure, it is necessary to use a silicon film of 10 .mu.m or more thickness in order to prevent generation of hard X-rays from copper and chromium in the underlying copper alloy. Furthermore, when the silicon film is made thicker as described above, the silicon film becomes more liable to peel off because of its thickness. This is disadvantageous in that the assembled X-ray source would be mechanically weak and would not have sufficient thermal dissipation.
Finally, if silicon is adhered directly onto a copper alloy, then mutual diffusion would occur between the copper and the silicon, and eventually at about 550.degree. C. there occurs an eutectic reaction, resulting in a mixed structure consisting of silicon and an .eta.-phase, which is not desirable for a soft X-ray source.